Question

How many moles of NaH2PO4 and how many moles of H3PO4 are needed to make one liter of a 450. mM phosphate buffer with a pH of 2.6? The pKa's for phosphoric acid are 2.16, 7.21, and 12.32.

ph= pka + log (conj base/conj acid )

Answer 1

total no of mol of phosphatebuffer = M*V = 450*1 = 450 mmol

pH of phosphate buffer = pka1 + log[NaH2PO4/H3PO4]

pka1 of H3PO4 =2.16

[NaH2PO4] = x

[H3PO4] = 450-x

2.6 = 2.16 + log(x/(450-x))

x = 330.135

[NaH2PO4] = x = 330.135 mmol

                        = 0.33 mol

[H3PO4] = 450-x = 450-330.135 = 119.86 mmol

                             = 0.11986 mol